Low impact development (LID) municipal ordinances require on-site retention and treatment of a fraction of storm water that would be discharged offsite. The total maximum daily load limit (TMDL) requirement dictates the type and configuration of treatment elements for each site. Where soil and site landscaping allow, naturally occurring swales will be used. Some sites will, however, require installation of treatment structures either alone or in combination with naturally occurring swales to minimize project-life cost, using the minimum number of necessary treatment structures. When multi pre-treatment steps are necessary, the first treatment stage will be removal or reduction of total suspended solids (TSS) and total oil and grease (TOG). Any dissolved constituents adsorb on suspended solid will also be removed.
This treated storm water (herein filtrate) may be discharge directly to the selected infiltration stage (swale or infiltration structure) or be discharged to subsequent tertiary treatment stages for removal of fine particulate, bacteria, or specific dissolved constituents. Such tertiary treatment is before delivery to a detention structure or an infiltration structure. A detention structure is used when the effluent cannot be discharged offsite. An infiltration structure is used to infiltrate directly into suitable soils to recharge local groundwater or for various uses such as landscape watering.